Journal of Phytology 2009, 1(4): 249–254 ISSN: 2075-6240 © Journal of Phytology, 2009 Available Online: www.journal-phytology.com REGULAR ARTICLE INFLUENCE OF WATER STRESS ON SEED GERMINATION CHARACTERISTICS IN INVASIVE DIPLOTAXIS HARRA (FORSSK.) BOISS (BRASSICACEAE) IN ARID ZONE OF TUNISIA Mustapha Gorai*, Tahar Tlig, Mohamed Neffati Laboratoire d’Ecologie Pastorale, Institut des Régions Arides, 4119 Médenine, Tunisia SUMMARY Diplotaxis harra (Forssk.) Boiss (Brassicaceae) has a wide ecological and geographical amplitudes in Tunisia and grows under a variety of environmental conditions. A laboratory experiment was performed to evaluate the effect of water potential on seed germination and recovery responses after transfer to distilled water. The germination responses of seeds at 15°C in complete darkness were determined over a wide range of PEG-6000 solutions of different osmotic potentials: 0 to –1 MPa. Greatest germination was obtained in distilled water (71%), and increases in osmolality of solutions progressively inhibited seed germination, less than 10% of the seeds germinated at –0.8 MPa. No germination was observed for the treatment of –1 MPa. The rate of germination decreased as osmotic potential decreased. When seeds were transferred to distilled water after 20 days of water stress, the recovery of germination increased with an increase in pretransfer PEG-6000 treatments. Seeds subjected to high osmotic potential had higher recovery percentages. Keywords: Diplotaxis harra, Germination, Recovery, Temperature, Water stress M. Gorai et al. Influence of Water Stress on Seed Germination Characteristics in Invasive Diplotaxis harra (Forssk.) Boiss (Brassicaceae) in Arid Zone of Tunisia . J Phytol 1 (2009) 249-254 *Corresponding Author, Email: gorai_mustapha@yahoo.fr 1. Introduction In arid ecosystems, the establishment of species may differ in their life cycle (annual/perennial), life form (shrubs/ herbaceous), response to drought (tolerance/ avoidance), time of flowering, reproductive effort, seed dispersal, and germination behaviour. Plants in these ecosystems have developed complementary adaptations and survival strategies throughout the stages of their life cycles. Among these stages, the seed has the highest resistance to extreme environmental factors, whereas the seedling has the lowest (Gutterman, 2002). In extreme desert conditions, it is the pattern of germination response to environmental factors that determines whether a particular species can appear or not in a specific